Bruce Crower’s Six-Stroke Engine

Bruce Crower’s Six-Stroke Engine adds a couple strokes to the usual four — intake, compression, combustion, and exhaust:

Bruce Crower has lived, breathed and built hot engines his whole life. Now he’s working on a cool one — one that harnesses normally-wasted heat energy by creating steam inside the combustion chamber, and using it to boost the engine’s power output and also to control its temperature.

If the inside of the cylinder's hot enough, you don't need to ignite gasoline to get a rapidly expanding gas; you can just inject water and get steam. That, of course, cools the cylinder back down again.